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Re: DSP Possibilities ?



The only thing stopping this is our capability to build the appropriate
hardware and software. There have been hams willing to develop software for
modems based on the Motorola and Analog Devices evaluation cards, but the
codecs on the DSP cards are limited to 48 ksps and a 20 kHz signal
bandwidth. QPSK can support 1 bps per Hz of bandwidth, so 19.2 kbps will fit
in that bandwidth. The symbol rate is 9600 baud, so we get 5 samples per bit
cell which is about the minimum that can be tolerated. 10 samples per baud
would give better performance.

There are newer audio codecs that sample at 96 ksps, cost only $6.00, and
support a 40 kHz bandwidth so 38.4 kbps should be possible at minimal
expense. If there are enough people that are willing to solder codec and DSP
chips with 48 and 144 pins each and 0.02" lead spacings to PCBs, it would be
easy to make kits of parts available.

Of course, you could go to multilevel modulation. 16-QAM would double the
bit rate to 76.8 kbps and would require at least 6 dB more SNR or 4 times
the transmitter power to acheive the same error rate.

P3D could provide an interesting platform for worldwide digital
communication. The uplink is probably not an issue -- the amount of power
available on the downlink will be what limits speeds. I've made some
calculations based on the information on the AMSAT Web pages.

The 435 Mhz transmitter puts out 250 watts PEP or 60 watts average power
into a phased array of 6 patch antennas which have a gain of about 13 dBic.
2/3 of the power is allocated to the "analog" passband which is 200 kHz
wide, so we have about +6 dBm/Hz EIRP at the satellite. The path loss at 435
MHz is about 179 dB and if the receiving antenna gain is +17dBic, -156
dBm/Hz is available at the receiver. If the receiving system noise figure is
1 dB (75K) and the sky noise is 50K, the noise power at the receiver is -177
dBm/Hz. This gives a SNR of 21 dB which should be adequate for 4-level
modulation. 16-QAM would support a data rate of 2 bps/Hz before error
correction. With an SSB radio and 2 kHz bandwidth, this would allow 4 kbps.
A 30 kHz bandwidth would be about the maximum allowable and could support 60
kbps.

73,

John
KD6OZH

----- Original Message -----
From: "Matt Donohoe" <TheRelic@dingoblue.net.au>
To: <amsat-bb@AMSAT.Org>
Sent: Wednesday, 20 September 2000 05:58 UTC
Subject: [amsat-bb] DSP Possibilities ?


> I am wondering using current DSP chips if speeds like 56k7, 63kbit etc
could be reached using modulation schemes such as QAM/QPSK ?
>
> What would be stopping this from happening ?
>
> ----
> Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
> To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
>

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